A two-in-one shampoo has the dual functions of cleaning and conditioning hair at the same time. Hair cleaning and conditioning are two processes that operate on entirely different principles. They are expected to provide different benefits to the hair, and they require ingredients with significantly different properties.
Functioning as a shampoo, a two-in-one should provide sufficient detergent actions to remove all the "unwanted" hair soil, which consists of hair lipids [1-7], dust particles , skin debris  from the hair shaft, and all other residues from previous hair cosmetics, such as hair spray. Shampoo removal of hair soil is a complex process [10-20], requiring the use of ingredients that are highly surface active. The cleaning agents should be able to wet the hair surface thoroughly. They should have the required properties to help emulsify and to solubilize the hair soil to facilitate its separation from the hair.
Shampooing also requires the production of desirable lathering characteristics that users tend to equate with hair cleaning. Effective shampoo ingredients must be able to develop a dense and copious lather. They must be highly water soluble and rinse off easily, without leaving any residues on the hair.
The materials that meet these requirements are anionic surfactants. Anionic surfactants are substances that carry a negative ion and contain both an oleophobic moiety and a hydrophilic moiety in the same molecule. They are soluble in water and are highly surface active. In aqueous solutions, at a concentration above the so-called critical micelle concentration (cmc), they tend to form micellar structures that are believed to be essential to the solubilization and emulsification processes that pull the soil particles from the hair surface. For these reasons, anionic surfactants are the cleaning ingredients of choice in shampoo formulations.
Amphoteric surfactants are also used in shampoos to some extent. These are compounds that have two different ionic sites on the same molecule. One of the ionic sites, which is cationic, can be amino nitrogen or a quaternary compound. The anionic site is either a sulfate, a carboxylate, or a sulfonate . The surfactant as a whole can be cationic, anionic, or zwitterionic, depending on the pH of the medium. The amphoteric surfactants generally do not clean as well or lather as effectively as anionic surfactants. But they are found to be milder [22,24], and interestingly, when mixed with anionic surfactants, they are tend to act synergisti-cally to lower the level of irritations of the latter . |
Hair cleaning action does have some consequences on hair aesthetics. Thoroughly cleaned hair has both wanted and unwanted attributes. Shampoo removes the hair soil and leaves the hair shiny and lustrous. It restores hair body. But thoroughly clean hair also feels raspy and harsh. Additionally, it is difficulty to comb through. It is prone to static buildup that tends to generate excessive flyaway, making the hair hard to manage.
2.2 Hair Conditioning Aspects
Hair conditioning is, in a sense, hair cleaning in reverse. In contrast to shampooing, which involves the removal of materials from the hair surface, conditioning is the action of putting back onto the hair an appropriate amount of suitable ingredients. While shampoo relies on the actions of anionic materials, hair conditioning requires the use of materials that are different in every aspect in terms of ionic characters, surface properties, and solubility characteristics.
One requirement of an effective hair conditioning agent is a proper degree of substantivity to the hair. The conditioning agent in a two-in-one shampoo must be able to survive the detergent actions of the cleaning agents as well as the subsequent rinsing actions. Another requirement is that the conditioning agent be able to impart to the hair the desirable tactile properties. Hair conditioning is expected to make the hair feel soft and smooth, to make the hair easy to comb, and to prevent static buildup. The amount of conditioning residues must not be excessive, which can create the phenomenon of "overconditioning," causing limp, dull-looking hair that is difficult to style.
Effective hair conditioning agents are in general cationic. They are more oleophobic, and are much less soluble in water. The ones that are commonly used are quaternary ammonium compounds, cationic polymers, and silicon oil. The substantivity of cationic materials is due to strong ionic interactions between the positive charges and the negatively charged hair surface. The substantivity of sili-cone oil, on the other hand, is a result of hydrophobic interactions. Being oleo-phobic, these conditioning agents also tend to suppress foam production and to have adverse effects on the lathering characteristics of anionic surfactants, complicating the task of formulating the the two-in-one shampoos.
Quaternary ammonium compounds are materials that contain at least one nitrogen atom with four alkyl or aryl groups attached to it. These compounds are always cationic regardless of the pH of the medium, and for this reason are in general not compatible with anionic surfactants.
Cationic polymers are materials made by attaching quaternized fatty alkyl groups to synthetic polymers or to modified natural polymers. Some quaternized polymers are extremely substantive to hair surface because of the high charge density. They are compatible or incompatible with anionic surfactants depending on the structures of the polymers and more importantly on the proportions at which the two are combined.
Silicone oils are also good, effective hair conditioning agents. They are highly hydrophobic and sparingly soluble in water. They are typically incompatible with anionic surfactants and need to be emulsified in a special way.
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